Oxadiazole derivative, and light-emitting element, light-emitting device, electronic device, and lighting device using the oxadiazole derivative

ABSTRACT

An object of one embodiment of the present invention is to provide a novel oxadiazole derivative as a substance having high excitation energy, in particular, a substance having high triplet excitation energy. One embodiment of the present invention is an oxadiazole derivative represented by General Formula (G1) below. 
     
       
         
         
             
             
         
       
     
     In General Formula (G1), R 1  represents either an alkyl group having 1 to 4 carbon atoms or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. In General Formula (G1), R 21  to R 27  separately represent any one of a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. In General Formula (G1), α represents a substituted or unsubstituted arylene group having 6 to 13 carbon atoms. In General Formula (G1), Z represents either a sulfur atom or an oxygen atom.

This application is a divisional of application Ser. No. 13/297,937filed on Nov. 16, 2011 (now U.S. Pat. No. 8,563,740 issued Oct. 22,2013) which claims priority under 35 USC 119 of JP 2010-257739 filed onNov. 18, 2010 in Japan, all which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxadiazole derivative, and alight-emitting element, a light-emitting device, an electronic device,and a lighting device each using the oxadiazole derivative.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements utilizing electroluminescence (EL).In the basic structure of such a light-emitting element, a layercontaining a light-emitting substance is interposed between a pair ofelectrodes. By voltage application to this element, light emission fromthe light-emitting substance can be obtained.

Since such light-emitting elements are self-luminous elements, it hasadvantages over liquid crystal displays in having high pixel visibilityand eliminating the need for backlights, for example; thus,light-emitting elements are thought to be suitable for flat paneldisplay elements. Light-emitting elements are also highly advantageousin that they can be thin and lightweight. Furthermore, very high speedresponse is also one of the features of such elements.

Furthermore, since such light-emitting elements can be formed in a filmform, they make it possible to provide planar light emission; thus,large-area elements utilizing planar light emission can be easilyformed. This is a difficult feature to obtain with point light sourcestypified by incandescent lamps and LEDs or linear light sources typifiedby fluorescent lamps. Thus, light-emitting elements also have greatpotential as planar light sources applicable to lighting devices and thelike.

Light-emitting elements utilizing electroluminescence are broadlyclassified according to whether they use an organic compound or aninorganic compound as a light-emitting substance. In the case where anorganic compound is used as a light-emitting substance, by applicationof voltage to a light-emitting element, electrons and holes are injectedinto a layer containing the light-emitting organic compound from a pairof electrodes, whereby current flows. Then, these carriers (i.e.,electrons and holes) are recombined, whereby the light-emitting organiccompound is excited. The light-emitting organic compound returns to theground state from the excited state, thereby emitting light. Note thatthe excited state of an organic compound can be a singlet excited stateor a triplet excited state, and luminescence from the singlet excitedstate (S*) is referred to as fluorescence, and luminescence from thetriplet excited state (T*) is referred to as phosphorescence. Thestatistical generation ratio thereof in a light-emitting element isconsidered to be S*:T*=1:3.

At room temperature, a compound that is capable of converting energy ofa singlet excited state into luminescence (hereinafter, referred to as afluorescent compound) exhibits only luminescence from the singletexcited state (fluorescence), not luminescence from the triplet excitedstate (phosphorescence). Thus, the internal quantum efficiency (theratio of generated photons to injected carriers) of a light-emittingelement using a fluorescent compound is assumed to have a theoreticallimit of 25% on the basis of S*:T*=1:3.

In contrast, with a compound that can convert energy of a tripletexcited state into luminescence (hereinafter, called a phosphorescentcompound), the internal quantum efficiency can be increased to 75% to100% in theory. In other words, an element using a phosphorescentcompound can have three to four times as high emission efficiency asthat of an element using a fluorescent compound. For these reasons, alight-emitting element using a phosphorescent compound has been activelydeveloped in recent years in order to achieve a highly-efficientlight-emitting element (e.g., see Non-Patent Document 1).

When formed using the above-described phosphorescent compound, alight-emitting layer of a light-emitting element is often formed suchthat a phosphorescent compound is dispersed in a matrix of anothercompound in order to suppress concentration quenching or quenching dueto triplet-triplet annihilation in the phosphorescent compound. Here,the substance serving as a matrix is called a host material, and thesubstance dispersed in a matrix, such as a phosphorescent compound, iscalled a guest material.

A host material needs to have higher triplet excitation energy (anenergy difference between a ground state and a triplet excited state)than a phosphorescent compound in the case where the phosphorescentcompound is a guest material. In addition, the host material needs tohave a carrier transport property by which desired carrier balance canbe controlled in a light-emitting layer. With the use of such a hostmaterial, characteristics of a light-emitting element can be improved.

REFERENCE

-   [Non-Patent Document 1] M. A. Baldo, and four others, Applied    Physics Letters, vol. 75, No. 1, 4-6 (1999)

SUMMARY OF THE INVENTION

A novel oxadiazole derivative is provided as a substance having highexcitation energy, in particular, a substance having high tripletexcitation energy. A novel oxadiazole derivative having a highelectron-transport property is provided. By applying the noveloxadiazole derivative to a light-emitting element, elementcharacteristics of the light-emitting element are improved. Alight-emitting device, an electronic device, and a lighting device eachhaving low power consumption and low driving voltage are provided.

One embodiment of the present invention is an oxadiazole derivativerepresented by General Formula (G1) below.

In General Formula (G1), R¹ represents either an alkyl group having 1 to4 carbon atoms or a substituted or unsubstituted aryl group having 6 to13 carbon atoms. In General Formula (G1), R²¹ to R²⁷ separatelyrepresent any one of a hydrogen atom, an alkyl group having 1 to 4carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. In General Formula (G1), α represents a substituted orunsubstituted arylene group having 6 to 13 carbon atoms. In GeneralFormula (G1), Z represents either a sulfur atom or an oxygen atom.

One embodiment of the present invention is an oxadiazole derivativerepresented by General Formula (G2) below.

In General Formula (G2), R²¹ to R²⁷ separately represent any one of ahydrogen atom, an alkyl group having 1 to 4 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms. InGeneral Formula (G2), α and β separately represent a substituted orunsubstituted arylene group having 6 to 13 carbon atoms. In GeneralFormula (G2), Z represents either a sulfur atom or an oxygen atom.

One embodiment of the present invention is an oxadiazole derivative inwhich α and β in General Formulae (G1) and (G2) separately representeither a substituted or unsubstituted phenylene group or a substitutedor unsubstituted biphenyldiyl group.

One embodiment of the present invention is an oxadiazole derivative inwhich α and β in General Formulae (G1) and (G2) separately represent asubstituted or unsubstituted phenylene group.

One embodiment of the present invention is an oxadiazole derivative inwhich α and β in General Formulae (G1) and (G2) separately represent anyone of structures represented by Structural Formulae (1-1) to (1-15)below.

One embodiment of the present invention is an oxadiazole derivativerepresented by General Formula (G1-1) below.

In General Formula (G1-1), R¹ represents either an alkyl group having 1to 4 carbon atoms or a substituted or unsubstituted aryl group having 6to 13 carbon atoms. In General Formula (G1-1), R²¹ to R²⁷ separatelyrepresent any one of a hydrogen atom, an alkyl group having 1 to 4carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. In General Formula (G1-1), Z represents either a sulfuratom or an oxygen atom.

One embodiment of the present invention is an oxadiazole derivativerepresented by General Formula (G2-1) below.

In General Formula (G2-1), R²¹ to R²⁷ separately represent any one of ahydrogen atom, an alkyl group having 1 to 4 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms. InGeneral Formula (G2-1), Z represents either a sulfur atom or an oxygenatom.

One embodiment of the present invention is an oxadiazole derivative inwhich R¹ in General Formulae (G1) and (G1-1) separately represent anyone of structures represented by Structural Formulae (2-1) to (2-22)below.

One embodiment of the present invention is an oxadiazole derivative inwhich R²¹ to R²⁷ in General formulae (G1), (G2), (G1-1), and (G2-1)separately represent any one of structures represented by StructuralFormulae (3-1) to (3-23) below.

One embodiment of the present invention is a light-emitting elementincluding an EL layer between a pair of electrodes, in which the ELlayer includes any of the above oxadiazole derivatives.

Note that since the oxadiazole derivatives which are embodiments of thepresent invention are suitable for use as a host material of alight-emitting layer in an EL layer because of their high excitationenergy. Accordingly, one embodiment of the present invention is alight-emitting element including an EL layer between a pair ofelectrodes. A light-emitting layer in the EL layer includes any of theabove oxadiazole derivatives which are embodiments of the presentinvention and a light-emitting substance.

The oxadiazole derivatives which are embodiments of the presentinvention have a high electron-transport property; therefore, theoxadiazole derivative is optimal for use as an electron-transportmaterial for an electron-transport layer in an EL layer of alight-emitting element.

One embodiment of the present invention is a light-emitting devicemanufactured using the light-emitting element which is one embodiment ofthe present invention.

One embodiment of the present invention is an electronic devicemanufactured using the light-emitting device which is one embodiment ofthe present invention.

One embodiment of the present invention is a lighting devicemanufactured using the light-emitting device which is one embodiment ofthe present invention.

Note that a light emitting device in this specification refers to animage display device, a light emitting device, or a light source.Further, the light-emitting device includes, in its category, all of amodule in which a connector such as a flexible printed circuit (FPC), atape automated bonding (TAB) tape, or a tape carrier package (TCP) isattached to a light-emitting device; a module having a TAB tape or a TCPprovided with a printed wiring board at the end thereof; and a modulehaving an integrated circuit (IC) directly mounted over a light-emittingelement by a chip on glass (COG) method.

According to one embodiment of the present invention, an oxadiazolederivative having high excitation energy, in particular, an oxadiazolederivative having high triplet excitation energy can be provided.According to one embodiment of the present invention, a light-emittingelement having high current efficiency, which is formed using theoxadiazole derivative of one embodiment of the present invention, can beprovided. According to one embodiment of the present invention, alight-emitting device, an electronic device, and a lighting device eachhaving low power consumption and low driving voltage, to which thelight-emitting elements are applied, can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B each illustrate a light-emitting element of oneembodiment of the present invention.

FIG. 2 illustrates a light-emitting element of one embodiment of thepresent invention.

FIGS. 3A and 3B illustrate a light-emitting device of one embodiment ofthe present invention.

FIGS. 4A and 4B illustrate a light-emitting device of one embodiment ofthe present invention.

FIGS. 5A to 5E illustrate electronic devices and a lighting device ofembodiments of the present invention.

FIG. 6 illustrates electronic devices of embodiments of the presentinvention.

FIG. 7 illustrates electronic devices and lighting devices ofembodiments of the present invention.

FIG. 8 illustrates an element structure of a light-emitting element inExample 5.

FIGS. 9A and 9B are NMR charts of DBTO11-II (abbreviation).

FIGS. 10A and 10B each show an absorption spectrum of DBTO11-II(abbreviation).

FIGS. 11A and 11B each show an emission spectrum of DBTO11-II(abbreviation).

FIGS. 12A and 12B are NMR charts of mDBTO11-II (abbreviation).

FIGS. 13A and 13B each show an absorption spectrum of mDBTO11-II(abbreviation).

FIGS. 14A and 14B each show an emission spectrum of mDBTO11-II(abbreviation).

FIGS. 15A and 15B are NMR charts of DBTO11-III (abbreviation).

FIGS. 16A and 16B each show an absorption spectrum of DBTO11-III(abbreviation).

FIGS. 17A and 17B each show an emission spectrum of DBTO11-III(abbreviation).

FIGS. 18A and 18B are NMR charts of DBT2O11-II (abbreviation).

FIGS. 19A and 19B each show an absorption spectrum of DBT2O11-II(abbreviation).

FIGS. 20A and 20B each show an emission spectrum of DBT2O11-II(abbreviation).

FIG. 21 shows current density vs. luminance characteristics ofLight-Emitting Elements 1 and 2.

FIG. 22 shows voltage vs. luminance characteristics of Light-EmittingElements 1 and 2.

FIG. 23 shows luminance vs. current efficiency characteristics ofLight-Emitting Elements 1 and 2.

FIG. 24 shows voltage vs. current characteristics of Light-EmittingElements 1 and 2.

FIG. 25 shows emission spectra of Light-Emitting Elements 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention are described withreference to the accompanying drawings. Note that the present inventionis not limited to the description given below, and it will be easilyunderstood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beconstrued as being limited to the description in the followingembodiments.

Embodiment 1

In Embodiment 1, oxadiazole derivatives which are embodiments of thepresent invention will be described.

An oxadiazole derivative according to one embodiment of the presentinvention is represented by General Formula (G1).

In General Formula (G1), R¹ represents either an alkyl group having 1 to4 carbon atoms or a substituted or unsubstituted aryl group having 6 to13 carbon atoms. In General Formula (G1), R²¹ to R²⁷ separatelyrepresent any one of a hydrogen atom, an alkyl group having 1 to 4carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. In General Formula (G1), α represents a substituted orunsubstituted arylene group having 6 to 13 carbon atoms. In GeneralFormula (G1), Z represents either a sulfur atom or an oxygen atom.

An oxadiazole derivative according to one embodiment of the presentinvention is represented by General Formula (G2) below.

In General Formula (G2), R²¹ to R²⁷ separately represent any one of ahydrogen atom, an alkyl group having 1 to 4 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms. InGeneral Formula (G2), α and β separately represent a substituted orunsubstituted arylene group having 6 to 13 carbon atoms. In GeneralFormula (G2), Z represents either a sulfur atom or an oxygen atom.

In an oxadiazole derivative according to one embodiment of the presentinvention, α and β in General Formulae (G1) and (G2) separatelyrepresent either a substituted or unsubstituted phenylene group or asubstituted or unsubstituted biphenyldiyl group.

In an oxadiazole derivative according to one embodiment of the presentinvention, α and β in General Formulae (G1) and (G2) separatelyrepresent a substituted or unsubstituted phenylene group.

In General Formulae (G1) and (G2), as specific examples of structuresrepresented by α and β, structures represented by Structural Formulae(1-1) to (1-15) shown below can be given.

One embodiment of the present invention is an oxadiazole derivativerepresented by General Formula (G1-1) below.

In General Formula (G1-1), R¹ represents either an alkyl group having 1to 4 carbon atoms or a substituted or unsubstituted aryl group having 6to 13 carbon atoms. In General Formula (G1-1), R²¹ to R²⁷ separatelyrepresent any one of a hydrogen atom, an alkyl group having 1 to 4carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. In General Formula (G1-1), Z represents either a sulfuratom or an oxygen atom.

One embodiment of the present invention is an oxadiazole derivativerepresented by General Formula (G2-1) below.

In General Formula (G2-1), R²¹ to R²⁷ separately represent any one of ahydrogen atom, an alkyl group having 1 to 4 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms. InGeneral Formula (G2-1), Z represents either a sulfur atom or an oxygenatom.

As specific examples of a substituent represented by R¹ in GeneralFormulae (G1) and (G1-1), substituents represented by StructuralFormulae (2-1) to (2-22) below can be given.

As specific examples of substituents represented by R²¹ to R²⁷ inGeneral Formulae (G1), (G2), (G1-1), and (G2-1), substituentsrepresented by Structural Formulae (3-1) to (3-23) below can be given.

Specific examples of the oxadiazole derivative of one embodiment of thepresent invention, which is represented by General Formula (G1), includeoxadiazole derivatives represented by Structural Formulae (100) to(167), (200) to (235), (300) to (367), and (400) to (435). However, thepresent invention is not limited to these.

A variety of reactions can be applied to methods of synthesizing theoxadiazole derivatives of the present invention. For example, theoxadiazole derivative of one embodiment of the present invention, whichis represented by General Formula (G1) below, can be synthesized byperforming a synthesis reaction described below. Note that methods ofsynthesizing the oxadiazole derivatives of the present invention are notlimited to the following synthesis methods.

Method of Synthesizing Oxadiazole Derivative Represented by GeneralFormula (G1)

The oxadiazole derivative represented by General Formula (G1) can besynthesized as illustrated in Synthesis Scheme (A-1). Specifically, ahalide of an oxadiazole derivative (Compound 1) is coupled with anorganoboron compound or boronic acid of a dibenzofuran derivative or adibenzothiophene derivative (Compound 2) by a Suzuki-Miyaura couplingreaction, whereby the oxadiazole derivative (General Formula (G1)) ofthe present invention can be obtained.

In Synthesis Scheme (A-1), Z represents either a sulfur atom or anoxygen atom; R¹ represents either an alkyl group having 1 to 4 carbonatoms or a substituted or unsubstituted aryl group having 6 to 13 carbonatoms; R²¹ to R²⁷ separately represent any one of hydrogen, an alkylgroup having 1 to 4 carbon atoms, and a substituted or unsubstitutedaryl group having 6 to 13 carbon atoms; and α represents a substitutedor unsubstituted arylene group having 6 to 13 carbon atoms. R⁵⁰ and R⁵¹separately represent either hydrogen or an alkyl group having 1 to 6carbon atoms. In Synthesis Scheme (A-1), R⁵⁰ and R⁵¹ may be bonded toeach other to form a ring. Furthermore, X¹ represents halogen,preferably bromine or iodine.

Examples of a palladium catalyst that can be used in Synthesis Scheme(A-1) are, but not limited to, palladium(II) acetate,tetrakis(triphenylphosphine)palladium(0),bis(triphenylphosphine)palladium(II) dichloride, and the like.

Examples of a ligand of the palladium catalyst that can be used inSynthesis Scheme (A-1) are, but not limited to,tri(ortho-tolyl)phosphine, triphenylphosphine, tricyclohexylphosphine,and the like.

Examples of a base that can be used in Synthesis Scheme (A-1) are, butnot limited to, an organic base such as sodium tert-butoxide, aninorganic base such as potassium carbonate or sodium carbonate, and thelike.

Examples of a solvent that can be used in Synthesis Scheme (A-1) are,but not limited to, the following solvents: a mixed solvent of tolueneand water; a mixed solvent of toluene, alcohol such as ethanol, andwater; a mixed solvent of xylene and water; a mixed solvent of xylene,alcohol such as ethanol, and water; a mixed solvent of benzene andwater; a mixed solvent of benzene, alcohol such as ethanol, and water; amixed solvent of water and an ether such as ethylene glycol dimethylether; and the like. In particular, a mixed solvent of toluene andwater, a mixed solvent of toluene, ethanol, and water, or a mixedsolvent of water and ether such as ethylene glycol dimethyl ether ispreferred.

As a coupling reaction in Synthesis Scheme (A-1), the Suzuki-Miyauracoupling reaction using the organoboron compound or the boronic acidrepresented by Compound 2 may be replaced with a cross coupling reactionusing an organoaluminum compound, an organozirconium compound, anorganozinc compound, an organotin compound, or the like. However, thepresent invention is not limited thereto. Further, in this coupling, atriflate group or the like may be used other than halogen; however, thepresent invention is not limited thereto.

Further, in the Suzuki-Miyaura coupling reaction illustrated inSynthesis Scheme (A-1), an organoboron compound or boronic acid of anoxadiazole derivative may be coupled with a halide of a dibenzofuranderivative or a dibenzothiophene derivative or with a dibenzofuranderivative or a dibenzothiophene derivative which has a triflate groupas a substituent by a Suzuki-Miyaura coupling reaction.

The oxadiazole derivative of one embodiment of the present invention,which is represented by General Formula (G2) below, can be synthesizedby performing a synthesis reaction described below. Note that methods ofsynthesizing the oxadiazole derivatives of the present invention are notlimited to the following synthesis methods.

Method of Synthesizing Oxadiazole Derivative Represented by GeneralFormula (G2)

The oxadiazole derivative represented by General Formula (G2) can besynthesized as illustrated in Synthesis Scheme (A-2). Specifically, ahalide of an oxadiazole derivative (Compound 3) is coupled with anorganoboron compound or boronic acid of a dibenzofuran derivative or adibenzothiophene derivative (Compound 4) by a Suzuki-Miyaura couplingreaction, whereby the oxadiazole derivative (General Formula (G2)) ofthe present invention can be obtained.

In Synthesis Scheme (A-2), Z represents either a sulfur atom or anoxygen atom; R²¹ to R²⁷ separately represent any one of hydrogen, analkyl group having 1 to 4 carbon atoms, and a substituted orunsubstituted aryl group having 6 to 13 carbon atoms; and α and βseparately represent a substituted or unsubstituted arylene group having6 to 13 carbon atoms. R⁵² and R⁵³ separately represent either hydrogenor an alkyl group having 1 to 6 carbon atoms. In Synthesis Scheme (A-2),R⁵² and R⁵³ may be bonded to each other to form a ring. Furthermore, X²and X³ represent halogen, preferably bromine or iodine.

Examples of a palladium catalyst that can be used in Synthesis Scheme(A-2) are, but not limited to, palladium(II) acetate,tetrakis(triphenylphosphine)palladium(0),bis(triphenylphosphine)palladium(II) dichloride, and the like.

Examples of a ligand of the palladium catalyst that can be used inSynthesis Scheme (A-2) are, but not limited to,tri(ortho-tolyl)phosphine, triphenylphosphine, tricyclohexylphosphine,and the like.

Examples of a base that can be used in Synthesis Scheme (A-2) are, butnot limited to, an organic base such as sodium tert-butoxide, aninorganic base such as potassium carbonate or sodium carbonate, and thelike.

Examples of a solvent that can be used in Synthesis Scheme (A-2) are,but not limited to, the following solvents: a mixed solvent of tolueneand water; a mixed solvent of toluene, alcohol such as ethanol, andwater; a mixed solvent of xylene and water; a mixed solvent of xylene,alcohol such as ethanol, and water; a mixed solvent of benzene andwater; a mixed solvent of benzene, alcohol such as ethanol, and water; amixed solvent of water and an ether such as ethylene glycol dimethylether; and the like. In particular, a mixed solvent of toluene andwater, a mixed solvent of toluene, ethanol, and water, or a mixedsolvent of water and ether such as ethylene glycol dimethyl ether ispreferred.

As a coupling reaction in Synthesis Scheme (A-2), the Suzuki-Miyauracoupling reaction using the organoboron compound or the boronic acidrepresented by Compound 4 may be replaced with a cross coupling reactionusing an organoaluminum compound, an organozirconium compound, anorganozinc compound, an organotin compound, or the like. However, thepresent invention is not limited thereto. Further, in this coupling, atriflate group or the like may be used other than halogen; however, thepresent invention is not limited thereto.

Further, in the Suzuki-Miyaura coupling reaction illustrated inSynthesis Scheme (A-2), an organoboron compound or boronic acid of anoxadiazole derivative may be coupled with a halide of a dibenzofuranderivative or a dibenzothiophene derivative or with a dibenzofuranderivative or a dibenzothiophene derivative which has a triflate groupas a substituent by a Suzuki-Miyaura coupling reaction.

Embodiment 2

In Embodiment 2, a light-emitting element using any of the oxadiazolederivatives described in Embodiment 1 will be described as oneembodiment of the present invention with reference to FIGS. 1A and 1B.

The light-emitting element in Embodiment 2 includes a first electrodewhich functions as an anode, a second electrode which functions as acathode, and an EL layer provided between the first electrode and thesecond electrode. Note that the light-emitting element in Embodiment 2can provide light emission when voltage is applied to each electrode sothat the potential of the first electrode is higher than that of thesecond electrode.

In addition, the EL layer of the light-emitting element in Embodiment 2includes a first layer (hole-injection layer), a second layer(hole-transport layer), a third layer (light-emitting layer), a fourthlayer (electron-transport layer), and a fifth layer (electron-injectionlayer), from the first electrode side.

In the light-emitting element in Embodiment 2 described in FIG. 1A or1B, a substrate 101 is used as a support of the light-emitting element.For the substrate 101, for example, glass, quartz, plastic, or the likecan be used. A flexible substrate can also be used. The flexiblesubstrate is a substrate that can be bent, such as a plastic substratemade of polycarbonate, polyarylate, or polyether sulfone, for example.Alternatively a film (made of polypropylene, polyester, polyvinylfluoride, polyvinyl chloride, or the like), an inorganic film formed byevaporation, or the like can be used. Note that a substrate other thanthese can be used as long as it can function as a support in amanufacturing process of the light-emitting element.

Note that the above substrate 101 may remain in a light-emitting deviceor an electronic device which is a product utilizing the light-emittingelement of one embodiment of the present invention, but may onlyfunctions as the support in its manufacturing process without remainingin an end product.

For a first electrode 102 fanned over the substrate 101, any of metals,alloys, conductive compounds, mixtures thereof, and the like which has ahigh work function (specifically, a work function of 4.0 eV or more) ispreferably used. Specific examples include indium oxide-tin oxide (ITO:indium tin oxide), indium oxide-tin oxide containing silicon or siliconoxide, indium oxide-zinc oxide (IZO: indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide (IWZO), and the like. Films ofthese conductive metal oxides are usually formed by sputtering, but maybe formed by application of a sol-gel method or the like. For example,an indium oxide-zinc oxide (IZO) film can be formed by a sputteringmethod using a target obtained by adding zinc oxide to indium oxide at 1wt % to 20 wt %. Further, a film of indium oxide containing tungstenoxide and zinc oxide (IWZO) can be formed by a sputtering method using atarget in which tungsten oxide and zinc oxide are added to indium oxideat 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %, respectively. Otherexamples are gold (Au), platinum (Pt), nickel (Ni), tungsten (W),chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu),palladium (Pd), titanium (Ti), a nitride of a metal material (e.g.,titanium nitride), and the like.

The first electrode 102 including any of these materials is usuallyformed by a sputtering method, but may be formed by a vacuum evaporationmethod, a CVD method, a coating method, an ink jet method, a printingmethod, a spin coating method, or the like.

Further, in the EL layer 103 formed over the first electrode 102, when afirst layer 111 formed in contact with the first electrode 102 is formedusing a later described composite material formed by combining anorganic compound and an electron acceptor (acceptor), as a substanceused for the first electrode 102, any of a variety of metals, alloys,and conductive compounds, a mixture thereof, or the like can be usedregardless of the work function; for example, aluminum (Al), silver(Ag), an alloy containing aluminum (e g., Al—Si), or the like can alsobe used.

For the EL layer 103 formed over the first electrode 102, a knownsubstance can be used, and either a low molecular compound or a highmolecular compound can be used. Note that substances forming the ELlayer 103 may consist of organic compounds or may include an inorganiccompound as a part.

The EL layer 103 can be formed by stacking an appropriate combination ofa hole-injection layer that includes a substance having a highhole-injection property, a hole-transport layer that includes asubstance having a high hole-transport property, a light-emitting layerthat includes a light-emitting substance, an electron-transport layerthat includes a substance having a high electron-transport property, anelectron-injection layer that includes a substance having a highelectron-injection property, and the like. In FIGS. 1A and 1B, an ELlayer is described in which the first layer (hole-injection layer) 111,a second layer (hole-transport layer) 112, a third layer (light-emittinglayer) 113, a fourth layer (electron-transport layer) 114, and a fifthlayer (electron-injection layer) 115 are stacked in this order from thefirst electrode 102 side.

The first layer 111 which is the hole-injection layer is a layerincluding a substance having a high hole-injection property. Examples ofa substance having a high hole-injection property that can be used aremolybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide,ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide,tantalum oxide, silver oxide, tungsten oxide, and manganese oxide.Besides, as a low molecular organic compound, a phthalocyanine-basedcompound such as phthalocyanine (abbreviation: H₂Pc), copper(II)phthalocyanine (abbreviation: CuPc), or vanadyl phthalocyanine(abbreviation: VON) can be used.

Other examples of a substance that can be used are aromatic aminecompounds which are low molecular organic compounds, such as4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1).

Still other examples of a substance that can be used are high molecularcompounds (e.g., oligomers, dendrimers, and polymers), such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD), and high molecular compounds to which acid is added, such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),or polyaniline/poly(styrenesulfonic acid) (PAni/PSS).

For the first layer 111, a composite material formed by combining anorganic compound and an electron acceptor (acceptor) may be used. Such acomposite material, in which holes are generated in the organic compoundby the electron acceptor, has an excellent hole-injection andhole-transport properties. In this case, the organic compound ispreferably a material excellent in transporting the generated holes (asubstance having a high hole-transport property). Note that when such acomposite material is used, a material for forming the first electrode102 can be selected regardless of its work function. In other words,besides a material with a high work function, a material with a low workfunction can also be used for the first electrode 102. In the case wherethe first layer 111 is formed using such a composite material, anorganic compound and an electron acceptor (acceptor) may beco-evaporated.

Examples of the organic compound used for the composite material are avariety of compounds such as aromatic amine compounds, carbazolederivatives, aromatic hydrocarbons, and high molecular compounds (e.g.,oligomers, dendrimers, and polymers). The organic compound used for thecomposite material is preferably organic compounds having a highhole-transport property, and specifically preferably a substance havinghole mobility of 10⁻⁶ cm²/Vs or more. Note that other than thesesubstances, any substance that has a property of transporting more holesthan electrons may be used. Organic compounds that can be used for thecomposite material will be specifically described below.

Examples of an organic compound that can be used for the compositematerial are aromatic amine compounds such as MTDATA, TDATA, DPAB,DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD), andN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and carbazole derivatives such as4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene (abbreviation: CzPA), and1,4-bis[4-(N-carbazolyl)phenyl-2,3,5,6-tetraphenylbenzene.

Other examples of an organic compound that can be used are aromatichydrocarbon compounds such as 2-tert-butyl-9,10-di(2-naphthyl)anthracene(abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene, and2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene.

Other examples of an organic compound that can be used are aromatichydrocarbon compounds, such as2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), and 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA).

Further, examples of the electron acceptor (acceptor) are organiccompounds such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane(abbreviation: F₄-TCNQ) and chloranil, transition metal oxides, andoxides of metals that belong to Groups 4 to 8 in the periodic table.Specific preferred examples include vanadium oxide, niobium oxide,tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide,manganese oxide, and rhenium oxide because their electron-acceptorproperties are high. Among these, molybdenum oxide is especiallypreferable since it is stable in the air, has a low hygroscopicproperty, and is easily treated.

The composite material may be formed using the above-described electronacceptor (acceptor) and the above-described high molecular compound suchas PVK, PVTPA, PTPDMA, or Poly-TPD and used for the first layer 111.

The second layer 112 which is the hole-transport layer is a layer thatincludes a substance having a high hole-transport property. As thesubstance having a high hole-transport property, particularly as a lowmolecular organic compound, an aromatic amine compound such as NPB (orα-NPD), TPD,4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), or4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB) can be used. The substances mentioned here aremainly substances that have hole mobility of 10⁻⁶ cm²/Vs or more. Notethat other than the above substances, any substance that has a propertyof transporting more holes than electrons may be used. Further, thelayer including a substance having a high hole-transport property is notlimited to a single layer, and may be a stack of two or more layersincluding any of the above substances.

Alternatively, for the second layer 112, a composite material in whichan electron acceptor (acceptor) is included in the above-describedsubstance having a high hole-transport property may be used.

For the second layer 112, a high molecular compound such as PVK, PVTPA,PTPDMA, or Poly-TPD can be used.

The third layer 113 which is the light-emitting layer is a layerincluding a light-emitting substance. As the light-emitting substance,for example, a fluorescent compound which emits fluorescence or aphosphorescent compound which emits phosphorescence can be used. Notethat in this embodiment, the case where the oxadiazole derivative whichis one embodiment of the present invention is used for thelight-emitting layer is described. For the light-emitting layer in whicha substance having a high light-emitting property (a guest material) isdispersed in another substance (a host material), the oxadiazolederivative which is one embodiment of the present invention can be usedas the host material.

In the case where the oxadiazole derivative which is one embodiment ofthe present invention is used as the host material and a substance whichemits fluorescence is used as the guest material in the light-emittinglayer, it is preferable to use, as the guest material, a substance whoselowest unoccupied molecular orbital level (LUMO level) is lower andhighest occupied molecular orbital level (HOMO level) is higher thanthose of the oxadiazole derivative. Examples of materials for blue lightemission includeN,N-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), and the like. Examples of materials for greenlight emission includeN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), and the like. Examples of materials for yellowlight emission include rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),and the like. Examples of materials for red light emission includeN,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD), and the like.

Alternatively, in the case where the oxadiazole derivative which is oneembodiment of the present invention is used as the host material and asubstance which emits phosphorescence is used as the guest material inthe light-emitting layer, it is preferable to use, as the guestmaterial, a substance having lower triplet excitation energy than theoxadiazole derivative. For example, the following organometalliccomplexes can be used, such asbis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate(abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)), and2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP).

Since the oxadiazole derivatives described which are embodiments of thepresent invention have an electron-transport property, using any of themin a light-emitting layer achieves its high electron-transport property.Such a light-emitting layer can provide light emission with highefficiency by using a guest material with high electron-trappingproperty.

In addition, as a substance (host material) in which a light-emittingsubstance (guest material) is dispersed, plural kinds of substances canbe used. Therefore, the light-emitting layer may include a second hostmaterial in addition to the oxadiazole derivatives which are embodimentsof the present invention. Note that as the second host material, a knownhost material can be used.

The fourth layer 114 which is the electron-transport layer is a layerwhich includes a substance having a high electron-transport property.Examples of the substance having a high electron-transport property usedfor the fourth layer 114 which can be used are: metal complexes having aquinoline skeleton or a benzoquinoline skeleton, such astris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂), andbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAIq); and the like. In addition, a metal complex having an oxazoleligand or a thiazole ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂), orthe like can also be used. In addition to the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can be used.Alternatively, a high molecular compound such aspoly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py) orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used.

Since the oxadiazole derivatives which are embodiments of the presentinvention have an electron-transport property, they can also be used foran electron-transport layer.

The substances described here are mainly substances having electronmobility of 10⁻⁶ cm²/Vs or more. Further, the electron-transport layeris not limited to a single layer and may be a stacked layer whichincludes two or more layers including the above-described substances.

The fifth layer 115 which is the electron-injection layer is a layerthat includes a substance having a high electron-injection property. Forthe fifth layer 115, an alkali metal, an alkaline earth metal, or acompound thereof, such as lithium fluoride (LiF), cesium fluoride (CsF),or calcium fluoride (CaF₂), can be used. Alternatively, a rare earthmetal compound such as erbium fluoride can be used.

Alternatively, a composite material in which an organic compound and anelectron donor (donor) are mixed may be used for the electron-injectionlayer. Such a composite material is excellent in an electron-injectionproperty and an electron-transport property because electrons aregenerated in the organic compound by the electron donor. In this case,the organic compound is preferably a material excellent in transportingthe generated electrons. Specifically, the above substances for formingthe electron-transport layer (e.g., a metal complex or a heteroaromaticcompound) can be used, for example. As the electron donor, a substanceexhibiting an electron-donating property to the organic compound may beused. Specifically, it is preferable to use an alkali metal, an alkalineearth metal, or a rare earth metal, such as lithium, cesium, magnesium,calcium, erbium, ytterbium, or the like. Further, it is preferable touse an alkali metal oxide or an alkaline earth metal oxide, such aslithium oxide, calcium oxide, or barium oxide. Alternatively, a Lewisbase such as magnesium oxide can also be used. Further alternatively, anorganic compound such as tetrathiafulvalene (abbreviation: TTF) can beused.

Note that the first layer (hole-injection layer) 111, the second layer(hole-transport layer) 112, the third layer (light-emitting layer) 113,the fourth layer (electron-transport layer) 114, and the fifth layer(electron-injection layer) 115 which are included in the above-describedEL layer 103 can each be formed by an evaporation method (including avacuum evaporation method), an ink-jet method, a coating method, or thelike. Note that a different formation method may be employed for eachlayer.

The second electrode 104 can be formed using a metal, an alloy, aconductive compound, a mixture thereof, or the like which has a low workfunction (specifically, a work function of 3.8 eV or less).Specifically, any of the following materials can be used: elements thatbelong to Group 1 or Group 2 in the periodic table, that is, alkalimetals such as lithium (Li) and cesium (Cs), alkaline earth metals suchas magnesium (Mg), calcium (Ca), and strontium (Sr), alloys thereof(e.g., Mg—Ag and Al—Li), rare earth metals such as europium (Eu) andytterbium (Yb), alloys thereof, and the like.

Note that in the case where a layer formed in contact with the secondelectrode 104 which is included in the EL layer 103 is formed using theabove-described composite material formed by combining the organiccompound and the electron donor (donor), any of a variety of conductivematerials such as aluminum, silver, ITO, indium oxide-tin oxidecontaining silicon or silicon oxide can be used regardless of the workfunction.

Note that the second electrode 104 can be formed by a vacuum evaporationmethod or a sputtering method. Alternatively, a coating method, anink-jet method, a spin coating method, or the like can be employeddepending on a material to be used.

In the above-described light-emitting element of the present invention,a current flows because of a potential difference generated between thefirst electrode 102 and the second electrode 104 and holes and electronsrecombine in the EL layer 103, so that light is emitted. Then, thisemitted light is extracted out through one or both of the firstelectrode 102 and the second electrode 104. Accordingly, one or both ofthe first electrode 102 and the second electrode 104 is/are an electrodehaving a light-transmitting property.

Note that the structure of the layer provided between the firstelectrode 102 and the second electrode 104 is not limited to the abovestructure. A structure other than the above may also be employed as longas a light-emitting region in which holes and electrons recombine isprovided in a portion away from the first electrode 102 and the secondelectrode 104 in order to prevent quenching due to proximity of thelight-emitting region to a metal.

In other words, a stacked structure of the layer is not particularlylimited, and a layer formed of a substance having a highelectron-transport property, a substance having a high hole-transportproperty, a substance having a high electron-injection property, asubstance having a high hole-injection property, a bipolar substance (asubstance having a high electron-transport property and a highhole-transport property), a hole-blocking material, or the like mayfreely be combined with a light-emitting layer.

Alternatively, as illustrated in FIG. 1B, a structure may be employed inwhich the second electrode 104 functioning as a cathode, the EL layer103, and the first electrode 102 functioning as an anode are stacked inthat order over the substrate 101. Note that the EL layer 103 in thiscase has a structure in which the fifth layer 115, the fourth layer 114,the third layer 113, the second layer 112, the first layer 111, and thefirst electrode 102 are stacked in that order over the second electrode104.

As described above, the light-emitting element can be manufactured usingthe oxadiazole derivative which is one embodiment of the presentinvention. Note that when the light-emitting element can be manufacturedusing the oxadiazole derivative which is one embodiment of the presentinvention, a light-emitting element having high emission efficiency canbe realized. In addition, a light-emitting element with long lifetimecan be obtained.

Note that with the use of the light-emitting element manufactured usingany of the oxadiazole derivatives which are embodiments of the presentinvention, a passive matrix light-emitting device or an active matrixlight-emitting device can be fabricated.

Note that there is no particular limitation on the structure of a TFT inthe case of manufacturing an active matrix light-emitting device. Forexample, a staggered TFT or an inverted staggered TFT can be used asappropriate. Further, a driver circuit formed over a TFT substrate maybe formed using both of an n-type TFT and a p-type TFT or only either ann-type TFT or a p-type TFT. Furthermore, there is no particularlimitation on the crystallinity of a semiconductor film used for theTFT. An amorphous semiconductor film may be used, or a crystallinesemiconductor film may be used.

Note that this embodiment can be used in combination with any of thestructures described in the other embodiments as appropriate.

Embodiment 3

In this embodiment, a light-emitting element having a stack of plural ELlayers of the light-emitting elements described in Embodiment 2(hereinafter, referred to as a stacked-type element) is described usingFIG. 2. This light-emitting element is a stacked-type light-emittingelement that has a plurality of EL layers (a first EL layer 203 and asecond EL layer 204) between a first electrode 201 and a secondelectrode 202. Note that although a structure in which two EL layers areformed is described in this embodiment, three or more EL layers may beformed.

In this embodiment, the first electrode 201 functions as an anode, andthe second electrode 202 functions as a cathode. Note that for the firstelectrode 201 and the second electrode 202, structures similar to thosedescribed in Embodiment 2 can be employed. Further, for the plurality ofEL layers (the first EL layer 203 and the second EL layer 204),structures similar to those described in Embodiment 2 can be employed.Note that structures of the first EL layer 203 and the second EL layer204 may be the same or different from each other and can be similar tothose described in Embodiment 2.

Further, a charge generation layer 205 is provided between the pluralityof EL layers (the first EL layer 203 and the second EL layer 204). Thecharge generation layer 205 has a function of injecting electrons intoone of the EL layers and injecting holes into the other of the EL layerswhen voltage is applied to the first electrode 201 and the secondelectrode 202. In this embodiment, when voltage is applied so that thepotential of the first electrode 201 is higher than that of the secondelectrode 202, the charge generation layer 205 injects electrons intothe first EL layer 203 and injects holes into the second EL layer 204.

Note that the charge generation layer 205 preferably has alight-transmitting property in terms of light extraction efficiency.Further, the charge generation layer 205 functions even when it haslower electric conductivity than the first electrode 201 or the secondelectrode 202.

The charge generation layer 205 may have either a structure including anorganic compound having a high hole-transport property and an electronacceptor (acceptor) or a structure including an organic compound havinga high electron-transport property and an electron donor (donor).Alternatively, both of these structures may be stacked. Note that theelectron acceptor and the electron donor are at least capable ofdonating and accepting electrons with the assistance of an electricfield.

In the case of the structure in which an electron acceptor is added toan organic compound having a high hole-transport property, examples ofthe substances having a high hole-transport property which can be usedinclude aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), and the like. The substances described here aremainly materials having hole mobility of 10⁻⁶ cm²/Vs or more. However,any substance other than the above substances may be used as long as itis a substance in which the hole-transport property is higher than theelectron-transport property.

In addition, examples of the electron acceptor include7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and transition metal oxides. Other examples areoxides of metals belonging to Group 4 to Group 8 in the periodic table.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable because of their high electron-acceptingproperties. Among these, molybdenum oxide is especially preferablebecause it is stable in the air, has a low hygroscopic property, and iseasily handled.

On the other hand, in the case of the structure in which an electrondonor is added to an organic compound having a high electron-transportproperty, examples of the substances having a high electron-transportproperty which can be used include a metal complex having a quinolineskeleton or a benzoquinoline skeleton, such astris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), and the like. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)₂),or the like can be used. Besides the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances described here are mainly substances having electron mobilityof 10⁻⁶ cm²/Vs or more. Note that any substance other than the abovesubstances may be used as long as it is a substance in which theelectron-transport property is higher than the hole-transport property.

Examples of the electron donor that can be used are alkali metals,alkaline earth metals, rare earth metals, metals that belong to Group 13in the periodic table and oxides or carbonates thereof, and preferablyspecifically lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca),ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, and thelike. An organic compound such as tetrathianaphthacene may be used asthe electron donor.

By forming the charge generation layer 205 with any of the abovematerials, it is possible to suppress an increase in driving voltagecaused when the EL layers are stacked.

Although the light-emitting element including two EL layers is describedin this embodiment, the embodiment can be applied to a light-emittingelement in which three or more EL layers are stacked. When a pluralityof EL layers with a charge generation layer interposed therebetween isarranged between a pair of electrodes, as in the light-emitting elementof this embodiment, it is possible to realize an element which can emitlight in a high luminance region while current density is kept low.Since the current density can be kept low, the element can have a longlifetime. When the light-emitting element is applied for illumination,voltage drop due to resistance of an electrode material can be reduced,whereby uniform light emission in a large area is possible. Moreover, alight-emitting device with low driving voltage and low power consumptioncan be achieved.

Furthermore, by making emission colors of the EL layers different, lighthaving a desired color can be obtained from the light-emitting elementas a whole. For example, the emission colors of first and second ELlayers are complementary in a light-emitting element including the twoEL layers, so that the light-emitting element can be made to emit whitelight as a whole. Note that the term “complementary” means colorrelationship in which an achromatic color is obtained when colors aremixed. That is, emission of white light can be obtained by mixture oflight emitted from substances whose emission colors are complementarycolors.

Further, the same can be applied to a light-emitting element includingthree EL layers. For example, the light-emitting element as a whole canemit white light when the emission color of the first EL layer is red,the emission color of the second EL layer is green, and the emissioncolor of the third EL layer is blue.

Note that Embodiment 3 can be combined with any of the structuresdescribed in Embodiments 1 and 2 as appropriate.

Embodiment 4

In Embodiment 4, a light-emitting device having a light-emitting elementof one embodiment of the present invention in a pixel portion will bedescribed with reference to FIGS. 3A and 3B. Note that FIG. 3A is a topview illustrating the light-emitting device, and FIG. 3B is across-sectional view taken along lines A-A′ and B-B′ of FIG. 3A.

In FIG. 3A, reference numeral 301 denotes a driver circuit portion (asource driver circuit), reference numeral 302 denotes a pixel portion,and reference numeral 303 denotes a driver circuit portion (a gatedriver circuit), which are each indicated by dotted lines. Referencenumeral 304 denotes a sealing substrate, reference numeral 305 denotes asealing material, and a portion enclosed by the sealing material 305 isa space 307.

Note that a lead wiring 308 is a wiring for transmitting signals thatare to be input to the source driver circuit 301 and the gate drivercircuit 303, and receives a video signal, a clock signal, a startsignal, a reset signal, and the like from an FPC (flexible printedcircuit) 309 which serves as an external input terminal. Although onlythe FPC is illustrated here, a printed wiring board (PWB) may beattached to the FPC. The light-emitting device in this specificationincludes not only a light-emitting device itself but also alight-emitting device to which an FPC or a PWB is attached.

Next, a cross-sectional structure will be described with reference toFIG. 3B. The driver circuit portion and the pixel portion are formedover an element substrate 310. Here, the source driver circuit 301 whichis the driver circuit portion and one pixel in the pixel portion 302 areillustrated. Note that, in the source driver circuit 301, a CMOS circuitwhich includes an n-channel TFT 323 and a p-channel TFT 324 is formed.The driver circuit may be any of a variety of circuits formed with TFTs,such as a CMOS circuit, a PMOS circuit, or an NMOS circuit. Although adriver-integrated type in which a driver circuit is formed over thesubstrate is described in this embodiment, the present invention is notlimited to this type, and the driver circuit can be formed outside thesubstrate.

The pixel portion 302 includes a plurality of pixels having a switchingTFT 311, a current control TFT 312, and a first electrode 313electrically connected to a drain of the current control TFT 312. Notethat an insulator 314 is formed to cover an end portion of the firstelectrode 313.

In order to improve coverage, the insulator 314 is preferably formed soas to have a curved surface with curvature at an upper end portion or alower end portion. For example, when positive photosensitive acrylic isused as a material for the insulator 314, only an upper end portion ofthe insulator 314 can have a curved surface with a radius of curvature(0.2 μm to 3 μm). For the insulator 314, it is also possible to useeither a negative type that becomes insoluble in an etchant by lightirradiation or a positive type that becomes soluble in an etchant bylight irradiation.

An EL layer 316 and a second electrode 317 are formed over the firstelectrode 313. Here, as a material for forming the first electrode 313,a material having a high work function is preferably used. For example,it is possible to use a single layer of an ITO film, an indium tin oxidefilm that includes silicon, an indium oxide film that includes zincoxide at 2 wt % to 20 wt %, a titanium nitride film, a chromium film, atungsten film, a Zn film, a Pt film, or the like, a stacked layer of atitanium nitride film and a film that mainly includes aluminum, athree-layer structure of a titanium nitride film, a film that mainlyincludes aluminum, and a titanium nitride film, or the like. Note thatwhen a stacked layer structure is employed, resistance of a wiring islow and a favorable ohmic contact is obtained.

In addition, the EL layer 316 is formed by any of various methods suchas an evaporation method using an evaporation mask, a dropletdischarging method like an ink-jet method, a printing method, and a spincoating method. The EL layer 316 includes any of the oxadiazolederivatives described in Embodiment 1. Further, another materialincluded in the EL layer 316 may be a low molecular material, anoligomer, a dendrimer, a high molecular material, or the like.

As the second electrode 317, any of a variety of metals, alloys, andelectrically conductive compounds, or a mixture thereof can be used.Among such materials, a metal, an alloy, an electrically conductivecompound, a mixture thereof, or the like having a low work function (awork function of 3.8 eV or less) is preferably used when the secondelectrode 317 is used as a cathode. As an example, an element belongingto Group 1 or Group 2 in the periodic table, i.e., an alkali metal suchas lithium (Li) or cesium (Cs), an alkaline earth metal such asmagnesium (Mg), calcium (Ca), or strontium (Sr), or an alloy containingany of these (e.g., Mg—Ag or Al—Li), and the like can be given.

In order that light generated in the EL layer 316 be transmitted throughthe second electrode 317, a stack of a metal thin film having a reducedthickness and a transparent conductive film (e.g., indium oxide-tinoxide (ITO), indium oxide-tin oxide that includes silicon or siliconoxide, indium oxide-zinc oxide (IZO), or indium oxide that includestungsten oxide and zinc oxide) can be used for the second electrode 317.

Further, the sealing substrate 304 is attached to the element substrate310 with the sealing material 305, so that a light-emitting element 318is provided in the space 307 enclosed by the element substrate 310, thesealing substrate 304, and the sealing material 305. The space 307 isfilled with a filler, and may be filled with an inert gas (such asnitrogen or argon) or the sealing material 305.

Note that an epoxy-based resin is preferably used as the sealingmaterial 305. Such a material preferably allows as little moisture andoxygen as possible to penetrate. As a material used for the sealingsubstrate 304, a plastic substrate formed of FRP (fiberglass-reinforcedplastics), PVF (polyvinyl fluoride), polyester, acrylic, or the like canbe used besides a glass substrate or a quartz substrate.

As described above, the active matrix light-emitting device having thelight-emitting element which is one embodiment of the present inventioncan be obtained.

Further, a light-emitting element of one embodiment of the presentinvention can be used for a passive matrix light-emitting device as wellas the above active matrix light-emitting device. FIGS. 4A and 4B are aperspective view and a cross-sectional view of a passive matrixlight-emitting device using a light-emitting element of one embodimentof the present invention. Note that FIG. 4A is a perspective view of thelight-emitting device, and FIG. 4B is a cross-sectional view taken alongline X-Y of FIG. 4A.

In FIGS. 4A and 4B, an EL layer 404 is provided between a firstelectrode 402 and a second electrode 403 over a substrate 401. An endportion of the first electrode 402 is covered with an insulating layer405. In addition, a partition layer 406 is provided over the insulatinglayer 405. The sidewalls of the partition layer 406 are aslope so that adistance between both the sidewalls is gradually narrowed toward thesurface of the substrate. In other words, a cross section taken alongthe direction of the short side of the partition layer 406 istrapezoidal, and the base (side facing in a direction similar to a planedirection of the insulating layer 405 and being in contact with theinsulating layer 405) is shorter than the upper side (side facing in thedirection similar to the plane direction of the insulating layer 405 andnot being in contact with the insulating layer 405). By providing thepartition layer 406 in such a way, a defect of a light-emitting elementdue to static electricity or the like can be prevented.

Thus, the passive matrix light-emitting device having a light-emittingelement which is one embodiment of the present invention can beobtained.

The light-emitting devices described in this embodiment (the activematrix light-emitting device and the passive matrix light-emittingdevice) are both formed using the light-emitting element which is oneembodiment of the present invention, and accordingly, the light-emittingdevices can have low power consumption.

Note that this embodiment can be combined with any of the structuresdescribed in the other embodiments as appropriate.

Embodiment 5

In this embodiment, with reference to FIGS. 5A to 5E, FIG. 6, and FIG.7, description is given of examples of a variety of electronic devicesand lighting devices that are completed by using a light-emitting devicewhich is one embodiment of the present invention.

Examples of the electronic devices to which the light-emitting device isapplied are television devices (also referred to as televisions ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cellular phones or cellular phonedevices), portable game machines, portable information terminals, audioreproducing devices, large game machines such as pachinko machines, andthe like. Specific examples of these electronic devices and a lightingdevice are illustrated in FIGS. 5A to 5E.

FIG. 5A illustrates an example of a television device. In a televisiondevice 7100, a display portion 7103 is incorporated into a housing 7101.The display portion 7103 is capable of displaying images, and thelight-emitting device can be used for the display portion 7103. Inaddition, here, the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Further, the remote controller 7110 may be provided with adisplay portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the receiver, general television broadcastingcan be received. Furthermore, when the television device 7100 isconnected to a communication network by wired or wireless connection viathe modem, one-way (from a transmitter to a receiver) or two-way(between a transmitter and a receiver, between receivers, or the like)data communication can be performed.

FIG. 5B illustrates a computer having a main body 7201, a housing 7202,a display portion 7203, a keyboard 7204, an external connection port7205, a pointing device 7206, and the like. This computer ismanufactured by using a light-emitting device for the display portion7203.

FIG. 5C illustrates a portable game machine having two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.A display portion 7304 is incorporated into the housing 7301 and adisplay portion 7305 is incorporated into the housing 7302. In addition,the portable game machine illustrated in FIG. 5C includes a speakerportion 7306, a recording medium insertion portion 7307, an LED lamp7308, an input means (an operation key 7309, a connection terminal 7310,a sensor 7311 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), or a microphone 7312), and thelike. It is needless to say that the structure of the portable gamemachine is not limited to the above as far as a light-emitting devicecan be used for at least either the display portion 7304 or the displayportion 7305, or both, and may include other accessories as appropriate.The portable game machine illustrated in FIG. 5C has a function ofreading out a program or data stored in a storage medium to display iton the display portion, and a function of sharing information withanother portable game machine by wireless communication. The portablegame machine illustrated in FIG. 5C can have a variety of functionswithout limitation to the above.

FIG. 5D illustrates an example of a cellular phone. A cellular phone7400 is provided with operation buttons 7403, an external connectionport 7404, a speaker 7405, a microphone 7406, and the like, in additionto a display portion 7402 incorporated into a housing 7401. Note thatthe cellular phone 7400 is manufactured using a light-emitting devicefor the display portion 7402.

When the display portion 7402 of the cellular phone 7400 illustrated inFIG. 5D is touched with a finger or the like, data can be input into thecellular phone 7400. Further, operations such as making a call andcreating e-mail can be performed by touch on the display portion 7402with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are mixed.

For example, in the case of making a call or creating e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be input. In this case, itis preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 7400, display on the screen of the display portion 7402can be automatically changed by determining the orientation of thecellular phone 7400 (whether the cellular phone is placed horizontallyor vertically for a landscape mode or a portrait mode).

The screen modes are switched by touch on the display portion 7402 oroperation with the operation buttons 7403 of the housing 7401.Alternatively, the screen modes can be switched depending on kinds ofimages displayed on the display portion 7402. For example, when a signalfor an image displayed on the display portion is data of moving images,the screen mode is switched to the display mode. When the signal is textdata, the screen mode is switched to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 7402 is detected and the input by touch on thedisplay portion 7402 is not performed during a certain period, thescreen mode may be controlled so as to be switched from the input modeto the display mode.

The display portion 7402 can function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, so thatpersonal authentication can be performed. Furthermore, by provision of abacklight or a sensing light source emitting a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan also be taken.

FIG. 5E illustrates an example of a lighting device. In a lightingdevice 7500, light-emitting devices 7503 a to 7503 d of one embodimentof the present invention are incorporated in a housing 7501 as lightsources. The lighting device 7500 can be attached to a ceiling, a wall,or the like.

The light-emitting device of one embodiment of the present inventionincludes a light-emitting element in a thin film form. Thus, when thelight-emitting device is attached to a base with a curved surface, thelight-emitting device with a curved surface can be obtained. Inaddition, when the light-emitting device is located in a housing with acurved surface, an electronic device or a lighting device with a curvedsurface can be obtained.

FIG. 6 illustrates a driver's seat and the periphery thereof inside avehicle. FIG. 6 illustrates an example in which a display device 600 isset on a dashboard and a display device 602 is set on a windshield. Inthe display device 600 illustrated in FIG. 6, a display portion 604 isincorporated in a housing with a curved surface and can display images.In the display device 600, the light-emitting device of one embodimentof the present invention can be used in the display portion 604.

In the display device 602 illustrated in FIG. 6, a display portion 606is incorporated in a housing with a curved surface and thelight-emitting device of one embodiment of the present invention can beused in the display portion 606. A pair of electrodes and a support of alight-emitting element which are included in the light-emitting deviceof one embodiment of the present invention are formed using alight-transmitting material, whereby light can be extracted through botha top surface and a bottom surface of the light-emitting device. Thus,the light-emitting device is used in the display portion 606, whereby auser can see the outside from the display portion 606 through thewindshield. Similarly, a user can also see an image displayed on thedisplay portion 606 from the outside through the windshield.

Note that the display device 600 or the display device 602 illustratedin FIG. 6 can also be used as a lighting device.

FIG. 7 illustrates an example in which the light-emitting device is usedas an indoor lighting device 701. Since the area of the light-emittingdevice can be increased, the light-emitting device can be used as alighting device with a large area. In addition, a lighting device 702 inwhich a light-emitting region has a curved surface can also be obtainedwith the use of a housing with a curved surface. A light-emittingelement included in the lighting device described in this embodiment isin a thin film form, which allows the housing to be designed morefreely. Therefore, the lighting device can be elaborately designed in avariety of ways.

A television device 7100 a that is a television device one example ofwhich is illustrated in FIG. 5A can be set in a room provided with thelighting device to which one embodiment of the present invention isapplied. The television device 7100 a may have a three-dimensionaldisplay function as well as a normal two-dimensional display function.In FIG. 7, a three-dimensional image can be watched with glasses 703 forwatching three-dimensional images.

As described above, the electronic devices and the lighting devices canbe obtained by application of the light-emitting device. Thelight-emitting device has a remarkably wide application range, and thuscan be applied to electronic devices in a variety of fields.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 4 asappropriate.

EXAMPLE 1

In Example 1, a method of synthesizing2-[4-dibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: DBTO11-II) represented by Structural Formula (100) inEmbodiment 1 is specifically described.

Synthesis of 2-[4-dibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(Abbreviation: DBTO11-II)

A synthesis scheme of2-[4-dibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: DBTO11-II) is illustrated in (B-1).

Into a 50-mL three neck flask were put 1.1 g (3.7 mmol) of2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole, 0.9 g (3.9 mmol) ofdibenzothiophene-4-boronic acid, and 56 mg (0.2 mmol) oftris(2-methylphenyl)phosphine, and the air in the flask was replacedwith nitrogen. To this mixture were added 3.7 mL of a 2.0M aqueouspotassium carbonate solution, 15 mL of toluene, and 5 mL of ethanol, andthe mixture was degassed by being stirred under reduced pressure.

To this mixture was added 8.3 mg (37 μmol) of palladium(II) acetate, andthe mixture was stirred at 80° C. for 7 hours under a nitrogen stream.After a predetermined time, the aqueous layer of the obtained mixturewas extracted with toluene. The obtained extract solution and theorganic layer were combined, washed with saturated brine, and dried overmagnesium sulfate. This mixture was separated by gravity filtration, andthe filtrate was concentrated to give a solid. This solid was purifiedby silica gel column chromatography.

The column chromatography was performed first using toluene as adeveloping solvent and then using a mixed solvent of a 20:1 ratio oftoluene to ethyl acetate as a developing solvent. The obtained fractionwas condensed to give a solid. This solid was purified by highperformance liquid column chromatography. The high performance liquidcolumn chromatography was performed using chloroform as a developingsolvent. The obtained fraction was condensed to give a solid. Hexane wasadded to this solid, followed by irradiation with ultrasonic waves. Thesolid was collected by suction filtration, so that 1.3 g of a whitepowder was obtained in 88% yield, which was the substance to beproduced.

By a train sublimation method, 1.3 g of the obtained white powder waspurified. The purification was performed under the following conditions:the temperature was 220° C.; the pressure was 2.5 Pa; and the flow rateof argon gas was 5 mL/min. Accordingly, 1.1 g of the compound wasobtained at a yield of 85%.

In addition, the compound obtained through the above synthesis methodwas measured by a nuclear magnetic resonance (NMR) method. Themeasurement data are shown below. ¹H NMR (CDCl₃, 300 MHz): δ(ppm)=7.46-7.63 (m, 7H), 7.84-7.89 (m, 1H), 7.94 (d, J=8.4 Hz, 2H),8.16-8.24 (m, 4H), 8.31 (d, J=8.4 Hz, 2H).

Further, ¹H NMR charts are shown in FIGS. 9A and 9B. Note that FIG. 9Bis a chart where the range of 7.0 ppm to 8.5 ppm in FIG. 9A is enlarged.It was found from the measurement results that2-[4-dibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: DBTO11-II) represented by Structural Formula (100) abovewas obtained.

Further, FIG. 10A shows an absorption spectrum of DBTO11-II(abbreviation) in a toluene solution, and FIG. 10B shows an absorptionspectrum of DBTO11-II (abbreviation) in a thin film. Anultraviolet-visible spectrophotometer (V-550, manufactured by JASCOCorporation) was used for the measurement. The absorption spectrum ofDBTO11-II (abbreviation) in the toluene solution was obtained bysubtracting the absorption spectra of quartz and toluene from theabsorption spectrum of DBTO11-II (abbreviation) in the toluene solutionin a quartz cell. In addition, the absorption spectrum of DBTO11-II(abbreviation) in the thin film was obtained by subtracting theabsorption spectrum of quartz from the absorption spectrum of a sampleformed by evaporating DBTO11-II (abbreviation) to a quartz substrate.

In FIG. 10A and FIG. 10B, the horizontal axis represents wavelength (nm)and the vertical axis represents absorption intensity (arbitrary unit).In the case of the toluene solution, absorption peaks were observed ataround 333 nm, 308 nm, and 292 nm, and in the case of the thin film,absorption peaks were observed at around 343 nm, 318 nm, 295 nm, 276 nm,and 233 nm.

Further, FIG. 11A shows an emission spectrum of DBTO11-II (abbreviation)in the toluene solution (excitation wavelength: 297 nm). FIG. 11B showsan emission spectrum of DBTO11-II (abbreviation) in the thin film(excitation wavelength: 340 nm). In FIGS. 11A and 11B, the horizontalaxis represents wavelength (nm) and the vertical axis representsemission intensity (arbitrary unit). The maximum emission wavelength was380 nm in the case of the toluene solution (excitation wavelength: 297nm), and 402 nm in the case of the thin film (excitation wavelength: 340nm).

Further, by measurement with a photoelectron spectrometer (AC-2,manufactured by Riken Keiki, Co., Ltd.) in the atmosphere, theionization potential of DBTO11-II (abbreviation) in the thin film wasfound to be 5.91 eV. As a result, the HOMO level was found to be −5.91eV. Furthermore, with the use of the absorption spectrum data ofDBTO11-II (abbreviation) in the thin film, the absorption edge wasobtained by a Tauc plot assuming direct transition. The absorption edgewas estimated as an optical energy gap, whereby the energy gap was 3.36eV. From the obtained value of the energy gap and the HOMO level, theLUMO level was −2.55 eV.

EXAMPLE 2

In Example 2, a method of synthesizing2-[3-dibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: mDBTO11-II) represented by Structural Formula (101) inEmbodiment 1 is specifically described.

Synthesis of 2-[3-dibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(Abbreviation: mDBTO11-II)

A synthesis scheme of2-[3-dibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: mDBTO11-II) is illustrated in (C-1).

Into a 50-mL three neck flask were put 1.1 g (3.7 mmol) of2-(3-bromophenyl)-5-phenyl-1,3,4-oxadiazole, 0.9 g (3.9 mmol) ofdibenzothiophene-4-boronic acid, and 56 mg (0.2 mmol) oftris(2-methylphenyl)phosphine, and the air in the flask was replacedwith nitrogen. To this mixture were added 3.7 mL of a 2.0M aqueouspotassium carbonate solution, 15 mL of toluene, and 5.0 mL of ethanol,and the mixture was degassed by being stirred under reduced pressure.

To this mixture was added 8.3 mg (37 mmol) of palladium(II) acetate, andthe mixture was stirred at 80° C. for 7 hours under a nitrogen stream.After a predetermined time, the aqueous layer of the obtained mixturewas extracted with toluene. The obtained extract solution and theorganic layer were combined, washed with saturated brine, and dried overmagnesium sulfate. This mixture was separated by gravity filtration, andthe filtrate was concentrated to give a solid. This solid was purifiedby silica gel column chromatography.

The column chromatography was performed first using toluene as adeveloping solvent and then using a mixed solvent of a 20:1 ratio oftoluene to ethyl acetate as a developing solvent. The obtained fractionwas condensed to give a solid. Methanol was added to this solid,followed by irradiation with ultrasonic waves. The solid was collectedby suction filtration, so that 1.3 g of a white powder was obtained in91% yield, which was the substance to be produced.

By a train sublimation method, 1.3 g of the obtained white powder waspurified. The purification was performed under the following conditions:the temperature was 230° C.; the pressure was 2.4 Pa; and the flow rateof argon gas was 5 mL/min. Accordingly, 1.1 g of the compound wasobtained at a yield of 85%.

In addition, the compound obtained through the above synthesis methodwas measured by a nuclear magnetic resonance (NMR) method. Themeasurement data are shown below. ¹H NMR (CDCl₃, 300 MHz): δ(ppm)=7.46-7.64 (m, 7H), 7.71 (t, J=7.8 Hz, 1H), 7.84-7.87 (m, 1H),7.94-7.98 (m, 1H), 8.15-8.26 (m, 5H), 8.50-8.51 (m, 1H).

Further, ¹H NMR charts are shown in FIGS. 12A and 12B. Note that FIG.12B is a chart where the range of 7.0 ppm to 9.0 ppm in FIG. 12A isenlarged. It was found from the measurement results that2-[3-dibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: mDBTO11-II) represented by Structural Formula (101) abovewas obtained.

Further, FIG. 13A shows an absorption spectrum of mDBTO11-II(abbreviation) in a toluene solution, and FIG. 13B shows an absorptionspectrum of mDBTO11-II (abbreviation) in a thin film. Anultraviolet-visible spectrophotometer (V-550, manufactured by JASCOCorporation) was used for the measurement. The absorption spectrum ofmDBTO11-II (abbreviation) in the toluene solution was obtained bysubtracting the absorption spectra of quartz and toluene from theabsorption spectrum of mDBTO11-II (abbreviation) in the toluene solutionin a quartz cell. In addition, the absorption spectrum of mDBTO11-II(abbreviation) in the thin film was obtained by subtracting theabsorption spectrum of quartz from the absorption spectrum of a sampleformed by evaporating mDBTO11-II (abbreviation) to a quartz substrate.

In FIG. 13A and FIG. 13B, the horizontal axis represents wavelength (nm)and the vertical axis represents absorption intensity (arbitrary unit).In the case of the toluene solution, absorption peaks were observed ataround 333 nm, 316 nm, and 286 nm, and in the case of the thin film,absorption peaks were observed at around 338 nm, 319 nm, 287 nm, 271 nm,and 243 nm.

Further, FIG. 14A shows an emission spectrum of mDBTO11-II(abbreviation) in the toluene solution (excitation wavelength: 290 nm).FIG. 14B shows an emission spectrum of mDBTO11-II (abbreviation) in thethin film (excitation wavelength: 285 nm). In FIGS. 14A and 14B, thehorizontal axis represents wavelength (nm) and the vertical axisrepresents emission intensity (arbitrary unit). The maximum emissionwavelength was 355 nm in the case of the toluene solution (excitationwavelength: 290 nm), and 370 nm in the case of the thin film (excitationwavelength: 285 nm).

Further, by measurement with a photoelectron spectrometer (AC-2,manufactured by Riken Keiki, Co., Ltd.) in the atmosphere, theionization potential of mDBTO11-II (abbreviation) in the thin film wasfound to be 5.63 eV. As a result, the HOMO level was found to be −5.63eV. Furthermore, with the use of the absorption spectrum data ofmDBTO11-II (abbreviation) in the thin film, the absorption edge wasobtained by a Tauc plot assuming direct transition. The absorption edgewas estimated as an optical energy gap, whereby the energy gap was 3.48eV. From the obtained value of the energy gap and the HOMO level, theLUMO level was −2.15 eV.

EXAMPLE 3

In Example 3, a method of synthesizing2-[4-(2,8-diphenyldibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: DBTO11-III) represented by Structural Formula (128) inEmbodiment 1 is specifically described.

Synthesis of2-[4-(2,8-diphenyldibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(Abbreviation: DBTO11-III)

A synthesis scheme of2-[4-(2,8-diphenyldibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: DBTO11-III)] is illustrated in (D-1).

Into a 50-mL three neck flask were put 0.8 g (2.7 mmol) of2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole, 1.1 g (2.9 mmol) of2,8-diphenyldibenzothiophene-4-boronic acid, and 41 mg (0.1 mmol) oftris(2-methylphenyl)phosphine, and the air in the flask was replacedwith nitrogen. To this mixture were added 3.0 mL of a 2.0M aqueouspotassium carbonate solution, 10 mL of toluene, and 3.4 mL of ethanol,and the mixture was degassed by being stirred under reduced pressure.

To this mixture was added 6.0 mg (27 μmol) of palladium(II) acetate, andthe mixture was stirred at 80° C. for 7 hours under a nitrogen stream.After a predetermined time, the aqueous layer of the obtained mixturewas extracted with chloroform. The obtained extract solution and theorganic layer were combined, washed with saturated brine, and dried overmagnesium sulfate. This mixture was separated by gravity filtration, andthe filtrate was concentrated to give an oily substance. This oilysubstance was purified by silica gel column chromatography.

The column chromatography was performed using toluene as a developingsolvent. The obtained fraction was condensed to give a solid. Methanolwas added to this solid, followed by irradiation with ultrasonic waves.The solid was collected by suction filtration, so that 1.4 g of a whitepowder was obtained in 93% yield, which was the substance to beproduced.

By a train sublimation method, 1.4 g of the obtained white powder waspurified. The purification was performed under the following conditions:the temperature was 310° C.; the pressure was 2.4 Pa; and the flow rateof argon gas was 5 mL/min. Accordingly, 1.2 g of the compound wasobtained at a yield of 86%.

In addition, the compound obtained through the above synthesis methodwas measured by a nuclear magnetic resonance (NMR) method. Themeasurement data are shown below. ¹H NMR (CDCl₃, 300 MHz): δ(ppm)=7.38-7.45 (m, 2H), 7.49-7.59 (m, 7H), 7.73-7.80 (m, 6H), 7.92 (d,J=8.4 Hz, 1H), 7.99 (d, J=8.1 Hz, 2H), 8.16-8.21 (m, 2H), 8.33 (d, J=8.1Hz, 2H), 8.45 (d, J=1.8 Hz, 2H).

Further, ¹H NMR charts are shown in FIGS. 15A and 15B. Note that FIG.15B is a chart where the range of 7.0 ppm to 8.5 ppm in FIG. 15A isenlarged. It was found from the measurement results that2-[4-(2,8-diphenyldibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: DBTO11-III) represented by Structural Formula (128) abovewas obtained.

Further, FIG. 16A shows an absorption spectrum of DBTO11-III(abbreviation) in a toluene solution, and FIG. 16B shows an absorptionspectrum of DBTO11-III (abbreviation) in a thin film. Anultraviolet-visible spectrophotometer (V-550, manufactured by JASCOCorporation) was used for the measurement. The absorption spectrum ofDBTO11-III (abbreviation) in the toluene solution was obtained bysubtracting the absorption spectra of quartz and toluene from theabsorption spectrum of DBTO11-III (abbreviation) in the toluene solutionin a quartz cell. In addition, the absorption spectrum of DBTO11-III(abbreviation) in the thin film was obtained by subtracting theabsorption spectrum of quartz from the absorption spectrum of a sampleformed by evaporating DBTO11-III (abbreviation) to a quartz substrate.

In FIG. 16A and FIG. 16B, the horizontal axis represents wavelength (nm)and the vertical axis represents absorption intensity (arbitrary unit).In the case of the toluene solution, absorption peaks were observed ataround 349 nm and 289 nm, and in the case of the thin film, absorptionpeaks were observed at around 360 nm and 279 nm.

Further, FIG. 17A shows an emission spectrum of DBTO11-III(abbreviation) in the toluene solution (excitation wavelength: 290 nm).FIG. 17B shows an emission spectrum of DBTO11-III (abbreviation) in thethin film (excitation wavelength: 330 nm). In FIGS. 17A and 17B, thehorizontal axis represents wavelength (nm) and the vertical axisrepresents emission intensity (arbitrary unit). The maximum emissionwavelength was 388 nm in the case of the toluene solution (excitationwavelength: 290 nm), and 412 nm in the case of the thin film (excitationwavelength: 330 nm).

Further, by measurement with a photoelectron spectrometer (AC-2,manufactured by Riken Keiki, Co., Ltd.) in the atmosphere, theionization potential of DBTO11-III (abbreviation) in the thin film wasfound to be 5.85 eV. As a result, the HOMO level was found to be −5.85eV. Furthermore, with the use of the absorption spectrum data ofDBTO11-III (abbreviation) in the thin film, the absorption edge wasobtained by a Tauc plot assuming direct transition. The absorption edgewas estimated as an optical energy gap, whereby the energy gap was 3.18eV. From the obtained value of the energy gap and the HOMO level, theLUMO level was −2.67 eV.

EXAMPLE 4

In Example 4, a method of synthesizing2,5-bis[4-(dibenzothiophen-4-yl)phenyl]-1,3,4-oxadiazole (abbreviation:DBT2O11-II) represented by Structural Formula (200) in Embodiment 1 isspecifically described.

Synthesis of 2,5-bis[4-(dibenzothiophen-4-yl)phenyl]-1,3,4-oxadiazole(Abbreviation: DBT2O11-II)

A synthesis scheme of2,5-bis[4-(dibenzothiophen-4-yl)phenyl]-1,3,4-oxadiazole (abbreviation:DBT2O11-II) is illustrated in (E-1).

Into a 50-mL three neck flask were put 1.1 g (2.9 mmol) of2,5-bis(4-bromophenyl)-1,3,4-oxadiazole, 1.4 g (6.1 mmol) ofdibenzothiophene-4-boronic acid, and 91 mg (0.3 mmol) oftris(2-methylphenyl)phosphine, and the air in the flask was replacedwith nitrogen. To this mixture were added 6.0 mL of a 2.0M aqueouspotassium carbonate solution, 12 mL of toluene, and 4.0 mL of ethanol,and the mixture was degassed by being stirred under reduced pressure.

To this mixture was added 13 mg (0.1 mmol) of palladium(II) acetate, andthe mixture was stirred at 80° C. for 7 hours under a nitrogen stream.After a predetermined time, the aqueous layer of the obtained mixturewas extracted with chloroform. The obtained extract solution and theorganic layer were combined, washed with saturated brine, and dried overmagnesium sulfate. This mixture was separated by gravity filtration, andthe filtrate was concentrated to give a solid. This solid was purifiedby silica gel column chromatography.

The column chromatography was performed using chloroform as a developingsolvent. The obtained fraction was condensed to give a solid. The solidwas recrystallized with toluene/hexane, so that 0.8 g of a white powderwas obtained in 53% yield, which was the substance to be produced.

By a train sublimation method, 0.8 g of the obtained white powder waspurified. The purification was performed under the following conditions:the temperature was 325° C.; the pressure was 2.4 Pa; and the flow rateof argon gas was 5 mL/min Accordingly, 0.7 g of the compound wasobtained at a yield of 88%.

In addition, the compound obtained through the above synthesis methodwas measured by a nuclear magnetic resonance (NMR) method. Themeasurement data are shown below. ¹H NMR (CDCl₃, 300 MHz): δ(ppm)=7.47-7.53 (m, 4H), 7.55-7.65 (m, 4H), 7.85-7.89 (m, 2H), 7.97 (d,J=9.0 Hz, 4H), 8.20-8.24 (m, 4H), 8.35 (d, J=8.7 Hz, 4H).

Further, ¹H NMR charts are shown in FIGS. 18A and 18B. Note that FIG.18B is a chart where the range of 7.0 ppm to 8.5 ppm in FIG. 18A isenlarged. It was found from the measurement results that2,5-bis[4-(dibenzothiophen-4-yl)phenyl]-1,3,4-oxadiazole (abbreviation:DBT2O11-II) represented by Structural Formula (200) above was obtained.

Further, FIG. 19A shows an absorption spectrum of DBT2O11-II(abbreviation) in a toluene solution, and FIG. 19B shows an absorptionspectrum of DBT2O11-II (abbreviation) in a thin film. Anultraviolet-visible spectrophotometer (V-550, manufactured by JASCOCorporation) was used for the measurement. The absorption spectrum ofDBT2O11-II (abbreviation) in the toluene solution was obtained bysubtracting the absorption spectra of quartz and toluene from theabsorption spectrum of DBT2O11-II (abbreviation) in the toluene solutionin a quartz cell. In addition, the absorption spectrum of DBT2O11-II(abbreviation) in the thin film was obtained by subtracting theabsorption spectrum of quartz from the absorption spectrum of a sampleformed by evaporating DBT2O11-II (abbreviation) to a quartz substrate.

In FIG. 19A and FIG. 19B, the horizontal axis represents wavelength (nm)and the vertical axis represents absorption intensity (arbitrary unit).In the case of the toluene solution, absorption peaks were observed ataround 342 nm, 339 nm, and 319 nm, and in the case of the thin film,absorption peaks were observed at around 347 nm, 329 nm, 297 nm, and 237nm.

Further, FIG. 20A shows an emission spectrum of DBT2O11-II(abbreviation) in the toluene solution (excitation wavelength: 344 nm).FIG. 20B shows an emission spectrum of DBT2O11-II (abbreviation) in thethin film (excitation wavelength: 344 nm). In FIGS. 20A and 20B, thehorizontal axis represents wavelength (nm) and the vertical axisrepresents emission intensity (arbitrary unit). The maximum emissionwavelengths were 390 nm and 375 nm in the case of the toluene solution(excitation wavelength: 344 nm), and 425 nm in the case of the thin film(excitation wavelength: 344 nm).

Further, by measurement with a photoelectron spectrometer (AC-2,manufactured by Riken Keiki, Co., Ltd.) in the atmosphere, theionization potential of DBT2O11-II (abbreviation) in the thin film wasfound to be 5.91 eV. As a result, the HOMO level was found to be −5.91eV. Furthermore, with the use of the absorption spectrum data ofDBT2O11-II (abbreviation) in the thin film, the absorption edge wasobtained by a Tauc plot assuming direct transition. The absorption edgewas estimated as an optical energy gap, whereby the energy gap was 3.26eV. From the obtained value of the energy gap and the HOMO level, theLUMO level was −2.65 eV.

EXAMPLE 5

In this example, described are a method for manufacturing alight-emitting element including any of the oxadiazole derivativesdescribed in Embodiment 1 as a host material of a light-emitting layerand measurement results of element characteristics. Specifically,Light-Emitting Element 1 formed using2,5-bis[4-(dibenzothiophen-4-yl)phenyl]-1,3,4-oxadiazole (abbreviation:DBT2O11-II) which is described in Example 4 and Light-Emitting Element 2formed using2-[4-(2,8-diphenyldibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: DBTO11-III) which is described in Example 3, will bedescribed.

Note that each element structure of the light-emitting elements of thisexample is illustrated in FIG. 8, in which a light-emitting layer 813 isformed using any of the oxadiazole derivatives which are embodiments ofthe present invention. Structural formulae of organic compounds used inthis example are shown below.

First, a film of indium tin oxide containing silicon oxide (ITSO) wasdeposited on a substrate 801 which was a glass substrate by a sputteringmethod to form a first electrode 802. The thickness was 110 nm and theelectrode area was 2 mm×2 mm. In this example, the first electrode 802was manufactured as an anode.

Next, an EL layer 803 in which a plurality of layers were stacked wasformed over the first electrode 802. In this example, the EL layer 803has a structure in which a first layer 811 which is a hole-injectionlayer, a second layer 812 which is a hole-transport layer, a third layer813 which is a light-emitting layer, a fourth layer 814 which is anelectron-transport layer, and a fifth layer 815 which is anelectron-injection layer are sequentially stacked.

The substrate 801 provided with the first electrode 802 was fixed on asubstrate holder that was provided in a vacuum evaporation apparatus sothat a surface provided with the first electrode 802 faced downward. Thepressure in the vacuum evaporation apparatus was reduced toapproximately 10⁻⁴ Pa. Then, on the first electrode 802,4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP)and molybdenum(VI) oxide were co-evaporated to form the first layer 811which is the hole-injection layer. The thickness of the first layer 811was 50 nm, and the evaporation rate was controlled so that the weightratio of BPAFLP to molybdenum oxide was 4:2 (=BPAFLP:molybdenum oxide).Note that the co-evaporation method refers to an evaporation method inwhich evaporation is carried out from a plurality of evaporation sourcesat the same time in one treatment chamber.

Next, a 10-nm-thick film of a hole-transport material was formed on thefirst layer 811 by an evaporation method with resistance heating to formthe second layer 812 which is the hole-transport layer. Note that forthe second layer 812, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) was used.

Next, the third layer 813 which is the light-emitting layer was formedover the second layer 812 by an evaporation method using resistanceheating. In the case where Light-Emitting Element 1 is formed, a firstfilm with a thickness of 20 nm was formed by co-evaporation of2,5-bis[4-(dibenzothiophen-4-yl)phenyl]-1,3,4-oxadiazole (abbreviation:DBT2O11-II) as a first host material,4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP) as a second host material, andtris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃)as a guest material. Note that the evaporation rate was controlled sothat the weight ratio of DBT2O11-II (abbreviation) to PCBA1BP(abbreviation) and Ir(ppy)₃ (abbreviation) was 1:0.25:0.08(=DBT2O11-II:PCBA1BP:Ir(ppy)₃).

Further, a second film with a thickens of 20 nm was formed byco-evaporation of2,5-bis[4-(dibenzothiophen-4-yl)phenyl]-1,3,4-oxadiazole (abbreviation:DBT2O11-II) as a host material andtris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃)as a guest material. Note that the evaporation rate was controlled sothat the weight ratio of DBT2O11-II (abbreviation) to Ir(ppy)₃(abbreviation) was 1:0.08 (=DBT2O11-II:Ir(ppy)₃). In other words, inLight-Emitting Element 1, the third layer 813 in which the first filmand the second film were stacked was formed.

In the case where Light-Emitting Element 2 is formed, a first film witha thickness of 20 nm was formed by co-evaporation of2-[4-(2,8-diphenyldibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: DBTO11-III) as a first host material,4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP) as a second host material, andtris(2-phenylpyridinato-N,C²)iridium(III) (abbreviation: Ir(ppy)₃) as aguest material. Note that the evaporation rate was controlled so thatthe weight ratio of DBTO11-III (abbreviation) to PCBA1BP (abbreviation)and Ir(ppy)₃ (abbreviation) was 1:0.25:0.08(=DBTO11-III:PCBA1BP:Ir(ppy)₃).

Further, a second film with a thickens of 20 nm was formed byco-evaporation of2-[4-(2,8-diphenyldibenzothiophen-4-yl)phenyl]-5-phenyl-1,3,4-oxadiazole(abbreviation: DBTO11-III) as a host material andtris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃)as a guest material. Note that the evaporation rate was controlled sothat the weight ratio of DBTO11-III (abbreviation) to Ir(ppy)₃(abbreviation) was 1:0.08 (=DBTO11-III:Ir(ppy)₃). In other words, inLight-Emitting Element 2, the third layer 813 in which the first filmand the second film were stacked was formed.

Furthermore, on the third layer 813, a 15-nm-thick film oftris(8-quinolinolato)aluminum(III) (abbreviation: Alq) and, thereon, a15-nm-thick film of bathophenanthroline (abbreviation: BPhen) wereformed to form the fourth layer 814 which is the electron-transportlayer.

On the fourth layer 814, the fifth layer 815 which is theelectron-injection layer was formed by depositing lithium fluoride (LiF)to a thickness of 1 nm.

Lastly, a 200-nm-thick film of aluminum was formed by an evaporationmethod using resistance heating to form a second electrode 804. Thus,Light-Emitting Elements 1 and 2 were formed.

Table 1 shows element structures of Light-Emitting Elements 1 and 2obtained through the above-described steps.

TABLE 1 First First Layer Second Layer Electrode (Hole-Injection(Hole-Transport Layer) 802 Layer) 811 812 Light-Emitting ITSOBPAFLP:MoOx BPAFLP Element 1 110 nm (=4:2) 10 nm 50 nm Light-EmittingITSO BPAFLP:MoOx BPAFLP Element 2 110 nm (=4:2) 10 nm 50 nm Third Layer(Light-Emitting Layer) 813 Light-Emitting DBT2O11II:PCBA1BP:Ir(ppy)₃DBT2O11II:Ir(ppy)₃ Element 1 (=1:0.25:0.08) (=1:0.08) 20 nm 20 nmLight-Emitting DBTO11III:PCBA1BP:Ir(ppy)₃ DBTO11III:Ir(ppy)₃ Element 2(=1:0.25:0.08) (=1:0.08) 20 nm 20 nm Fourth Layer Fifth Layer Second(Electron-Transport (Electron-Injection Electrode Layer) 814 Layer) 815804 Light-Emitting Alq BPhen LiF Al Element 1 15 nm 15 nm 1 nm 200 nmLight-Emitting Alq BPhen LiF Al Element 2 15 nm 15 nm 1 nm 200 nm*Mixture ratios are all represented in weight ratios.

Light-Emitting Elements 1 and 2 were sealed in a glove box under anitrogen atmosphere without being exposed to the air. Then, theoperation characteristics of these light-emitting elements weremeasured. Note that the measurement was carried out at room temperature(in an atmosphere kept at 25° C.).

Table 2 shows voltage (V), current density (mA/cm²), CIE chromaticitycoordinates (x, y), luminance (cd/m²), and current efficiency (cd/A) ofeach of Light-Emitting Elements 1 and 2 at a luminance of about 1000cd/m².

TABLE 2 Current Current Voltage Density Chromaticity LuminanceEfficiency (V) (mA/cm²) (x, y) (cd/m²) (cd/A) Light- 3.2 2.6 (0.34,0.61) 860 32.6 Emitting Element 1 Light- 3.0 1.6 (0.34, 0.61) 845 52.0Emitting Element 2

FIG. 21 shows current density vs. luminance characteristics ofLight-Emitting Elements 1 and 2; FIG. 22 shows voltage vs. luminancecharacteristics thereof; FIG. 23 shows luminance vs. current efficiencycharacteristics thereof; and FIG. 24 shows voltage vs. currentcharacteristics thereof. In FIG. 21, the vertical axis representsluminance (cd/m²) and the horizontal axis represents current density(mA/cm²). In FIG. 22, the vertical axis represents luminance (cd/m²) andthe horizontal axis represents voltage (V). In FIG. 23, the verticalaxis represents current efficiency (cd/A) and the horizontal axisrepresents luminance (cd/m²). In FIG. 24, the vertical axis representscurrent (mA) and the horizontal axis represents voltage (V).

From FIG. 23, the maximum current efficiency of Light-Emitting Element 1is 47 cd/A, and the maximum current efficiency of Light-Emitting Element2 is 56 cd/A. This demonstrates that the light-emitting elementincluding the oxadiazole derivative which is one embodiment of thepresent invention has extremely high efficiency.

FIG. 25 shows emission spectra of Light-Emitting Elements 1 and 2. Asshown in FIG. 25, in each case of Light-Emitting Elements 1 and 2, anemission wavelength derived from Ir(ppy)₃ (abbreviation) which was usedas the guest material was observed, whereas emission wavelengths derivedfrom the oxadiazole derivative which is one embodiment of the presentinvention (DBT2O11-II (abbreviation) or DBTO11-III (abbreviation)) whichwas used as the host material and PCBA1BP (abbreviation) which was usedas the second host material were not observed. Thus, it was confirmedthat the oxadiazole derivative which is one embodiment of the presentinvention served as the host material of the light-emitting layer of thelight-emitting element.

REFERENCE EXAMPLE

In this reference example, an example of synthesizing4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP)used as a material of Light-Emitting Element 1 will be described.

Step 1: Method of Synthesizing 9-(4-bromophenyl)-9-phenylfluorene

In a 100-mL three-neck flask, 1.2 g (50 mmol) of magnesium was heatedand stirred for 30 minutes under reduced pressure to be activated. Thiswas cooled to room temperature, and the flask was made to contain anitrogen atmosphere. Then, several drops of dibromoethane were added, sothat foam formation and heat generation were confirmed. After 12 g (50mmol) of 2-bromobiphenyl dissolved in 10 mL of dehydrated diethyl etherwas slowly added dropwise to this mixture, the mixture was heated andstirred under reflux for 2.5 hours, so that a Grignard reagent wasprepared.

Into a 500-mL three-neck flask were placed 10 g (40 mmol) of4-bromobenzophenone and 100 mL of dehydrated diethyl ether. After theGrignard reagent which was synthesized in advance was slowly addeddropwise to this mixture, the mixture was heated and stirred underreflux for 9 hours.

After reaction, this mixture solution was filtered to give a residue.The obtained residue was dissolved in 150 mL of ethyl acetate, and1N-hydrochloric acid was added to the mixture until it was made acid,which was then stirred for 2 hours. The organic layer of this liquid waswashed with water, and magnesium sulfate was added thereto to removemoisture. This suspension was filtered, and the obtained filtrate wasconcentrated to give a highly viscous substance. Into a 500-mL recoveryflask were placed this highly viscous substance, 50 mL of glacial aceticacid, and 1.0 mL of hydrochloric acid. The mixture was stirred andheated at 130° C. for 1.5 hours under a nitrogen atmosphere to bereacted.

After the reaction, this reaction mixture solution was filtered to givea residue. The obtained residue was washed with water, an aqueous sodiumhydroxide solution, water, and methanol in this order. Then, the mixturewas dried, so that the substance which was the object of the synthesiswas obtained as 11 g of a white powder in 69% yield. The synthesisscheme of Step 1 is illustrated in the following (J-1).

Step 2: Method of Synthesizing4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (Abbreviation: BPAFLP)

Into a 100-mL three-neck flask were placed 3.2 g (8.0 mmol) of9-(4-bromophenyl)-9-phenylfluorene, 2.0 g (8.0 mmol) of4-phenyl-diphenylamine, 1.0 g (10 mmol) of sodium tert-butoxide, and 23mg (0.04 mmol) of bis(dibenzylideneacetone)palladium(0), and the air inthe flask was replaced with nitrogen. Then, 20 mL of dehydrated xylenewas added to this mixture. After the mixture was degassed by beingstirred under reduced pressure, 0.2 mL (0.1 mmol) oftri(tert-butyl)phosphine (a 10 wt % hexane solution) was added to themixture. This mixture was stirred and heated at 110° C. for 2 hoursunder a nitrogen atmosphere to be reacted.

After the reaction, 200 mL of toluene was added to this reaction mixturesolution, and this suspension was filtered through Florisil (produced byWako Pure Chemical Industries, Ltd., Catalog No. 540-00135) and Celite(produced by Wako Pure Chemical Industries, Ltd., Catalog No.531-16855). The obtained filtrate was concentrated, and the resultingsubstance was purified by silica gel column chromatography (with adeveloping solvent of toluene and hexane in a 1:4 ratio). The obtainedfraction was concentrated, and acetone and methanol were added to themixture. The mixture was irradiated with ultrasonic waves and thenrecrystallized, so that the substance which was the object of thesynthesis was obtained as 4.1 g of a white powder in 92% yield. Thesynthesis scheme of Step 2 is illustrated in the following (J-2).

The Rf values of the substance that was the object of the synthesis,9-(4-bromophenyl)-9-phenylfluorene, and 4-phenyl-diphenylamine wererespectively 0.41, 0.51, and 0.27, which were found by silica gel thinlayer chromatography (TLC) (with a developing solvent of ethyl acetateand hexane in a 1:10 ratio).

This compound was identified as4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP),which was the object of the synthesis, by a nuclear magnetic resonance(NMR) method.

¹H NMR data of the obtained compound are shown below. ¹H NMR (CDCl₃, 300MHz): δ (ppm)=6.63-7.02 (m, 3H), 7.06-7.11 (m, 6H), 7.19-7.45 (m, 18H),7.53-7.55 (m, 2H), 7.75 (d, J=6.9, 211).

This application is based on Japanese Patent Application serial no.2010-257739 filed with Japan Patent Office on Nov. 18, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An oxadiazole derivative represented by GeneralFormula (G2),

wherein: R²¹ to R²⁷ separately represent a hydrogen atom, an alkyl grouphaving 1 to 4 carbon atoms, or a substituted or unsubstituted aryl grouphaving 6 to 13 carbon atoms; α and β separately represent a substitutedor unsubstituted arylene group having 6 to 13 carbon atoms; and Zrepresents a sulfur atom or an oxygen atom.
 2. The oxadiazole derivativeaccording to claim 1, wherein at least one of α and β represents asubstituted or unsubstituted biphenyldiyl group.
 3. The oxadiazolederivative according to claim 1, wherein at least one of α and βrepresents a substituted phenylene group.
 4. The oxadiazole derivativeaccording to claim 1, wherein at least one of α and β represents any oneof structures represented by Structural Formulae (1-1) to (1-15),


5. A light-emitting element comprising the oxadiazole derivativeaccording to claim
 1. 6. A light-emitting device comprising thelight-emitting element according to claim
 5. 7. A lighting devicecomprising the light-emitting device according to claim
 6. 8. Anelectronic device comprising the light-emitting device according toclaim
 6. 9. An oxadiazole derivative represented by General Formula(G2-1),

wherein: R²¹ to R²⁷ separately represent a hydrogen atom, an alkyl grouphaving 1 to 4 carbon atoms, or a substituted or unsubstituted aryl grouphaving 6 to 13 carbon atoms; and Z represents a sulfur atom or an oxygenatom.
 10. A light-emitting element comprising the oxadiazole derivativeaccording to claim
 9. 11. A light-emitting device comprising thelight-emitting element according to claim
 10. 12. A lighting devicecomprising the light-emitting device according to claim
 11. 13. Anelectronic device comprising the light-emitting device according toclaim 11.